Melting behavior in laser powder bed fusion revealed by in situ X-ray and thermal imaging

This study investigates the variation of the melting behavior with time during the laser-based powder bed fusion of metals (PBF-LB/M) process using in situ X-ray and thermal imaging. Ti-6Al-4V powder was irradiated by a 200-W fiber laser at one point for 1 s. We classified the melting behavior visually by analyzing the X-ray images and quantitatively evaluated the depression zone depth and length as characteristics of the melt pool and depression zone shapes. Consequently, we elucidated the variation in the melting type with time until the melt pool entered a steady state as follows. Immediately after starting the laser irradiation, the extension velocities of the depression zone length and depth are similar, but a keyhole is generated owing to a decrease in the expansion velocity of the depression zone length. Then, a spherical melt pool floats up and remains in that position. It is considered that the melt pool grows rapidly owing to inhaling the powder around the melt pool and becomes larger than the depression zone length at the height. Therefore, a pore is generated between the powder bed and the bottom of the melt pool. Finally, the melt pool becomes flattened most likely caused by the increasing wettability between the melt pool and around the melt pool. These phenomena occur even by one-point laser irradiation without laser scanning.